The Collaborative International Dictionary of English v.0.48:

Countable \Count"a*ble\ (-?-b'l), a.
   Capable of being numbered.
   [1913 Webster]

WordNet (r) 3.0 (2006):

countable
    adj 1: that can be counted; "countable sins"; "numerable assets"
           [syn: countable, denumerable, enumerable,
           numerable]

The Free On-line Dictionary of Computing (30 December 2018):

countable
countably many
uncountable

    A term describing a set which is isomorphic
   to a subet of the natural numbers.  A countable set has
   "countably many" elements.  If the isomorphism is stated
   explicitly then the set is called "a counted set" or "an
   enumeration".

   Examples of countable sets are any finite set, the natural
   numbers, integers, and rational numbers.  The real
   numbers and complex numbers are not [proof?].

   (1999-08-29)